In Phage, don't sit in a loop once we've read all messages from an agent

Summary: Ref T2794. The logic is intended to read "loop until we're done processing messages", but is currently missing a break;. This doesn't affect behavior yet, just makes us eat way too much CPU.

Test Plan: Saw time phage remote --hosts db001-16 status drop from 99% CPU time to <10%.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2794

Differential Revision: https://secure.phabricator.com/D17387